It is known in the art to combine two liquids using a venturi injector unit. A source of pressurized liquid is coupled to the input port of the venturi injector unit. This liquid creates a reduced pressure within the venturi chamber as it flows through the unit and exits via the venturi injector unit output port. If a source of second liquid is coupled to the venturi chamber, it will be drawn into the chamber by the reduced pressure. The result is that a mixture of the first and second liquids will be delivered at the venturi injector unit output port. Such a system is disclosed in U.S. Pat. No. 3,598,288 to Pomgate.
Although such a venturi injector unit system is straightforward and economical, it does not provide constant metering, i.e., the ratio of the liquids in the output mixture is not constant. The ratio varies, for example, with fluctuations in the throughflow rate of the first liquid, and with fluctuations in the back pressure at the output port of the venturi injector unit. These ratio variations can be somewhat minimized by providing a high pressure pump to transport the first liquid, and a lower pressure pump to transport the second liquid. Such an approach is disclosed in U.S. Pat. No. 4,217,924 to Finger. Although his system is undoubtedly an improvement, the conventional pumps used by Finger does not necessarily provide a constantly metered output mixture.
The problem of providing a controllably metered output mixture is compounded if the second liquid is a chemical because conventional pumps are subject to chemical attack. As the chemical causes deterioration of pump gears, pistons, diaphragms, and the like, the flow characteristics of the pump change, varying the output mixture ratio. What is required is an inexpensive system that can accurately meter a mixture that includes a liquid chemical.